The present invention relates to an apparatus for indexing the origin of a moving member to be driven with a stepper motor. This apparatus is applicable to the indexing of the origin of a carriage on a printer or typewriter.
The position of carriages on some of the conventional printers and typewriters is controlled incrementally by open-loop control using a stepper motor. In this incremental position control system, the origin of the carriage must be set in order to know its present position. To this effect, the arrival of the carriage at a predetermined position on the path of its movement is detected with the combination of a contactor and a limit switch or of a light-shielding element and a photointerrupter which are provided at said predetermined position and on the carriage. When the detector consisting of a light-shielding element and a photointerrupter (or a contactor and a limit switch) outputs an origin signal, the position of the carriage at that time is indexed as the origin of the carriage.
The above described detector depends on a mechanical contactor for detecting the position of the moving carriage relative to the fixed frame, so in the face of temperature changes or mechanical vibrations, the subtle relationship between the excited phases of a stepper motor and the timing of detection of an origin signal will vary and in each origin indexing operation, the excited phase of the stepper motor, or its rotational angle, occurring at the time when the origin signal is detected varies and fails to attain a constant value.
In order to solve these problems and to ensure more precise origin indexing by matching the origin to the excited phase at all times of origin indexing operations, a method depending on the principle depicted in FIG. 5(a) has been proposed. In this method, a four-phase excitation stepper motor is allowed to rotate counterclockwise toward the origin until an origin signal is produced, and thereafter, the motor is rotated with excitation being performed up to the first specified phase, say, phase D, and a position P7 corresponding to said specified excited phase D is indexed as the origin.
As already pointed out, the relationship between the origin signal as an output from the detector and the excited phase is subject to change on account of temperature changes or mechanical vibrations. Therefore, depending upon the timing of origin signal detection with respect to the specified excited phase D which corresponds to the origin P7, the origin indexing position determined by the system described above might shift by one cycle of excited, i.e. by four steps, either leftward or rightward with respect to the origin P7, leading to erroneous indexing of position P3 or P11 as the origin.
For instance, if position P7 is assumed to be the true origin in FIG. 5(a), it can be correctly indexed if the origin signal is detected within the range leftward of position P11 and rightward of position P7. On the other hand, if the origin signal is detected within the range leftward of position P7 and rightward of position P3, the latter position P3 is indexed as the origin, which is deviated by 4 steps leftward of the true origin P7. This means that if the timing of origin signal detection in the vicinity of position P7 changes even slightly to the left of position P7, an error of 4 steps can occur in the precision of origin indexing.
The true origin P7 can also be indexed if the origin signal is detected within the range leftward of position P11 and rightward of position P7. However, if the origin signal is detected within the range leftward of position P15 and rightward of position P11, the latter position P11 is indexed as the origin, which is deviated by 4 steps rightward of the true origin P7. This means that if the timing of original signal detection in the vicinity of position P11 changes even slightly to the right of position P11, an error of 4 steps can occur in the precision of origin indexing.
As described above, the prior art methods of origin indexing have had the disadvantage of low precision of indexing operations that result from the instability in the origin indexing position.